Originally designed as an upgrade to the medium tank M4 Sherman, the M26 Pershing first entered service in 1945. A series of prototypes led to the final design, but the overall development of the Pershing was hindered by Army Ground Forces opposition. The M26 was the US Army's first operational heavy tank, but it was unreliable and underpowered, so its service was limited (only 20 Pershings were deployed during WWII, and the model was withdrawn from Korea in favor of the more powerful M46). Though not in itself a success, the M26 gave rise to other more successful heavy tanks such as the M47, M48 and M60 Patton tanks.